A few people who regularly read and comment on my blog might call me to task for this unZen-like question; oh well. What insight do you wish had come to you earlier in life?
Aside from a few wayward years, I've largely been a fairly diligent student. And when I first began studying jazz guitar in the late 70's, I had the good fortune to study with the late Harry Leahey, arguably New Jersey's foremost jazz guitar teacher. At that point, though I'd been supporting myself with my guitar for several years and playing the instrument for over ten, playing jazz was new to me. With Harry's exacting & excellent instruction, I worked very hard to become what I understood a jazz guitarist should be. After Harry died, I continued my study with three other excellent teachers.
Fast forward to the summer of 2002. That year I realized a long-postponed goal and attended a weeklong National Guitar Workshop, enrolled in the jazz section. When featured instructor Pat Metheny, a giant in jazz guitar, talked about growing up loving the Beatles and how he used harmonic concepts he learned from them playing jazz, I felt the hair on my neck stand up. For all the years I'd studied and played jazz guitar, somehow I'd let myself put aside earlier musical concepts that weren't labelled "jazz". That included most of what I'd learned playing the Beatles, the Temptations, the Beach Boys, etc. from the time I was 14. In that flash of insight as Metheny spoke, I suddenly realized how I'd hindered my growth as a musician. No teacher from Harry on had ever laid this dogma on me; it was my invention.
I've spent the last ten years fully embracing all the music that is in me and integrating it all into my playing; I'm such a better musician for that. But I do wish I could have gotten this insight earlier. Would be very nice to know I'm not alone in that. If you share, I promise I will not contact the Zen police.